British operational military pilots have begun flying what is being touted as the world's most advanced stealth fighter jet, the F-35.

But even as they take to the skies, Defence Secretary Philip Hammond has given the clearest indication yet that the UK may not now buy all the jets it had planned.

Speaking exclusively to Sky News, Mr Hammond pledged that the first 48 aircraft on order at a cost of around £100m each would be bought to service the new Queen Elizabeth Class aircraft carriers from 2020.

But he would not commit to a further 90 planes, which had originally been proposed.

He said: "It's dependent on politics, money and the state of the world, but it's also dependent on what is not yet clearly known, what the mix between manned fighter jets and unmanned aircraft is going to be."

Mr Hammond said there were two trains of thought, one suggesting an 80/20 split of manned to unmanned aircraft in future, the other suggesting the exact opposite.

He said the final decision would determine how many manned F-35s the UK could buy.

The Defence Secretary interrupted a visit to Washington to meet his recently-appointed US counterpart Chuck Hagel and fly to the Naval Air Station at Patuxent River in Maryland where the F-35, or the Lightning II as it will be known in Britain, is being tested.

Mr Hammond looked on as former Harrier pilot Pete "Wizzer" Wilson, who now works for defence contractor BAE Systems, which is part of the project team for the F-35, demonstrated the 'B' variant of the aircraft, which is capable of a short take-off and vertical landing.

In Florida, at Eglin Air Force Base in the northwest of the state, the first UK military operational pilots began flying the F-35B just six weeks ago.

There are only two of them - Lt Commander Ian "Tidders" Tidball from the Royal Navy and Squadron Leader Frankie Buchler from the RAF.

Both have vast experience on other types of fast jets - Lt Cdr Tidball on Sea Harriers and F-18s, Sqn Ldr Bulcher on the Jaguar and latterly Typhoon aircraft.

After frustrating delays due to problems with the test aircraft and then the weather, both British pilots are now qualified to fly the F-35.

Lt Cdr Tidball told me after his qualifying flight: "Having come back from the Harrier and F-18, the aircraft handles absolutely exceptionally, very responsive even in these bumpy conditions."

Sqn Ldr Buchler is also a fan of the new jet, parts of which are still classified and Sky News was not allowed to film.

He said: "It's awesome, a boyhood dream. I've always wanted to fly the best fast jets in the world and here we are with a fantastic platform and I'm lucky enough to be one of the first guys from the UK flying it.

"It's better than working for a living."

Image:The new jet is designed to replace Britain's reliance on aging Harrier jets

The British pilots fly two of the three UK jets so far delivered. But it will be another five years before the first squadron, based at RAF Marham, is set up on UK soil.

In the meantime, 17 Squadron will be the first UK unit to fly the F-35B out of Edwards Air Force base in California from next year.

The F-35 programme comprises three variants in all.

The A model is the "base" model and will be flown predominantly by the US Air Force, the B variant, which the UK is buying, will also be flown by the US Marine Corps, while the C or carrier variant, is adapted with "cat and trap" equipment to fly on and off American aircraft carriers.

The bulk of the projected orders will come from the US - close to 2,500 planes - with the UK originally due to buy 138 in total and the other "partner" nations, Canada, Italy, the Netherlands, Turkey, Australia, Denmark and Norway, due to buy about 500 between them.

The projections from Lockheed Martin, the lead contractor, suggest that including sales to other "coalition" countries there will be 3,922 F-35s sold during its planned 30-year production run.

That's good news for the UK economy too, because 15% by value of each sale comes to these shores, largely through the contribution of BAE Systems in Lancashire, where much of the tail of every F-35 is manufactured.

The biggest danger to the programme at the moment is the immense cost, estimated at £2.5bn so far, given that defence budgets across the world - particularly in the West - are being slashed.